Glycine receptors

Overview

The inhibitory glycine receptor [nomenclature as agreed by the NC-IUPHAR sub-committee on glycine receptors] is a member of the Cys-loop superfamily of transmitter-gated ion channels that includes the zinc activated channels, GABAA, nicotinic acetylcholine and 5-HT3 receptors [16]. The receptor is expressed either as a homo-pentamer of α subunits, or a complex now thought to harbour 2α and 3β subunits [2,6], that contain an intrinsic anion channel. Four differentially expressed isoforms of the α-subunit (α1-α4) and one variant of the β-subunit (β1, GLRB, P48167) have been identified by genomic and cDNA cloning. Further diversity originates from alternative splicing of the primary gene transcripts for α1 (α1INS and α1del), α2 (α2A and α2B), α3 (α3S and α3L) and β (βΔ7) subunits and by mRNA editing of the α2 and α3 subunit [4,19,23]. Both α2 splicing and α3 mRNA editing can produce subunits (i.e., α2B and α3P185L) with enhanced agonist sensitivity. Predominantly, the mature form of the receptor contains α1 (or α3) and β subunits while the immature form is mostly composed of only α2 subunits. RNA transcripts encoding the α4-subunit have not been detected in adult humans. The N-terminal domain of the α-subunit contains both the agonist and strychnine binding sites that consist of several discontinuous regions of amino acids. Inclusion of the β-subunit in the pentameric glycine receptor contributes to agonist binding, reduces single channel conductance and alters pharmacology. The β-subunit also anchors the receptor, via an amphipathic sequence within the large intracellular loop region, to gephyrin. The latter is a cytoskeletal attachment protein that binds to a number of subsynaptic proteins involved in cytoskeletal structure and thus clusters and anchors hetero-oligomeric receptors to the synapse [11-12,21]. G-protein βγ subunits enhance the open state probability of native and recombinant glycine receptors by association with domains within the large intracellular loop [31-32]. Intracellular chloride concentration modulates the kinetics of native and recombinant glycine receptors [24]. Intracellular Ca2+ appears to increase native and recombinant glycine receptor affinity, prolonging channel open events, by a mechanism that does not involve phosphorylation [5].

Comments

Data in the table refer to homo-oligomeric assemblies of the α-subunit, significant changes introduced by co-expression of the β1 subunit are indicated in parenthesis. Not all glycine receptor ligands are listed within the table, but some that may be useful in distinguishing between glycine receptor isoforms are indicated (see detailed view pages for each subunit: α1, α2, α3, α4, β ). pregnenolone sulphate, tropisetron and colchicine, for example, although not selective antagonists of glycine receptors, are included for this purpose. strychnine is a potent and selective competitive glycine receptor antagonist with affinities in the range 5–15 nM. RU5135 demonstrates comparable potency, but additionally blocks GABAA receptors. There are conflicting reports concerning the ability of cannabinoids to inhibit [14], or potentiate and at high concentrations activate [1,3,7,27-28] glycine receptors. Nonetheless, cannabinoid analogues may hold promise in distinguishing between glycine receptor subtypes [28]. In addition, potentiation of glycine receptor activity by cannabinoids has been claimed to contribute to cannabis-induced analgesia relying on Ser296/307 (α1/α3) in M3 [27]. Several analogues of muscimol and piperidine act as agonists and antagonists of both glycine and GABAA receptors. picrotoxin acts as an allosteric inhibitor that appears to bind within the pore, and shows strong selectivity towards homomeric receptors. While its components, picrotoxinin and picrotin, have equal potencies at α1 receptors, their potencies at α2 and α3 receptors differ modestly and may allow some distinction between different receptor types [29]. Binding of picrotoxin within the pore has been demonstrated in the crystal structure of the related C. elegans GluCl Cys-loop receptor [8]. In addition to the compounds listed in the table, numerous agents act as allosteric regulators of glycine receptors (comprehensively reviewed in [13,15,26,33]). Zn2+ acts through distinct binding sites of high- and low-affinity to allosterically enhance channel function at low (<10 µM) concentrations and inhibits responses at higher concentrations in a subunit selective manner [20]. The effect of Zn2+ is somewhat mimicked by Ni2+. Endogenous Zn2+ is essential for normal glycinergic neurotransmission mediated by α1 subunit-containing receptors [9]. Elevation of intracellular Ca2+ produces fast potentiation of glycine receptor-mediated responses. Dideoxyforskolin (4 µM) and tamoxifen (0.2–5 µM) both potentiate responses to low glycine concentrations (15 µM), but act as inhibitors at higher glycine concentrations (100 µM). Additional modulatory agents that enhance glycine receptor function include inhalational, and several intravenous general anaesthetics (e.g.minaxolone, propofol and pentobarbitone) and certain neurosteroids. ethanol and higher order n-alcohols also enhance glycine receptor function although whether this occurs by a direct allosteric action at the receptor [18], or through βγ subunits [30] is debated. Recent crystal structures of the bacterial homologue, GLIC, have identified transmembrane binding pockets for both anaesthetics [22] and alcohols [10]. Solvents inhaled as drugs of abuse (e.g.toluene, 1-1-1-trichloroethane) may act at sites that overlap with those recognising alcohols and volatile anaesthetics to produce potentiation of glycine receptor function. The function of glycine receptors formed as homomeric complexes of α1 or α2 subunits, or hetero-oligomers of α1/β or α2/β subunits, is differentially affected by the 5-HT3 receptor antagonist tropisetron (ICS 205-930) which may evoke potentiation (which may occur within the femtomolar range at the homomeric glycine α1 receptor), or inhibition, depending upon the subunit composition of the receptor and the concentrations of the modulator and glycine employed. Potentiation and inhibition by tropeines involves different binding modes [17]. Additional tropeines, including atropine, modulate glycine receptor activity.